The collection of foul-smelling and often flammable bilge gases in boats presents a health and safety hazard. Boats often are equipped with electrically powered bilge pumps for the purpose of clearing the boat of explosive gases before an in-board engine is started. If a pilot does not remember to run the bilge pump prior to starting the motor, an explosion and fire may result. One effort to avoid this problem is described in U.S. Pat. No. 3,771,920 in which inflatable plastic bags are provided within liquid-containing compartments in a boat, the rocking of the boat (often absent in heavy, stable craft) serving to alternately inflate and deflate the bags to cause bilge gases to eventually be exhausted from the boat. This device would appear to be expensive, rather heavy and awkward to manage, and difficult to install, and the volume of bilge gases which are exhausted would appear to depend upon the volume of the plastic bags. Much to be desired is a lightweight, simple, inexpensive, easily installed and continuously operating bilge pump which does not depend for its operation upon electric power nor upon rocking of a boat.